Poly(ethylene aromatic carboxylate ester) resins such as polyethylene terephthalate, polyethylene naphthalate, polytrimethylene terephthalate and polytetramethylene terephthalate (which will be referred to as polyester resins hereinafter) have excellent mechanical properties, excellent heat resistance, excellent electric insulating properties and excellent chemical resistance and are widely used as materials for forming shaped articles such as fibers and bottle-formed articles in which the above-mentioned properties are utilized.
As the process for producing polyethylene terephthalate, for example, there is known a process of directly esterifying terephthalic acid with ethylene glycol, or transesterifying a lower alkylester of terephthalic acid, such as dimethyl terephthalate with ethylene glycol, or reacting terephthalic acid with ethylene oxide to form an ethylene glycol ester of terephthalic acid and/or a polymer having a low polymerization degree and polycondensing the reaction product with heating under reduced pressure until a predetermined polymerization degree is attained.
In the production of the polyester resin such as polyethylene terephthalate, generally a polycondensation catalyst is used in order to allow the polymerization reaction to smoothly proceed. The rate of the polycondensation reaction and the quality of the resulting polymer are drastically influenced according to the kind of the polycondensation catalyst. Various metal compounds are known as the polycondensation catalyst. Among these metal compounds, an antimony (Sb) compound such as antimony trioxide is widely used because it is cheap and has a high polymerization activity and also the resulting polymer has comparatively good color tone. However, when the Sb compound is used as the polymerization catalyst, a portion thereof is reduced during the polycondensation reaction to form metallic Sb or other foreign matters, and thus causing a problem in that the resulting polymer is darkened and/or the production process is made unstable, resulting in deterioration of the quality of the formed article produced from the resulting resin.
When the Sb compound is used as a polycondensation catalyst for polyester and the resulting polyester resin is continuously melt-spun for a long time, foreign matters (which may be referred to as spinneret foreign matters, hereinafter) are deposited and accumulated around spinning orifices and a bending phenomenon arises in molten polymer streams, and thus causing a problem in that the filaments are fuzzed and/or filament yarn breakages occur in the resulting filament yarn during the spinning and drawing steps.
As the polycondensation catalyst other than the antimony compound, a germanium compound and a titanium compound such as tetra-n-butoxytitanium are proposed. The germanium compound is considerably expensive and, therefore, there is a problem that a cost in the production of the polyester becomes higher. When using the titanium compound as the polycondensation catalyst, a phenomenon of the deposition of foreign matters around spinning orifices is suppressed during the melt spinning of the resulting polyester resin. However, in this case, there arises such a known problem peculiar to the titanium compound that the resulting polyester itself is colored yellow and/or the melt of the resulting polyester resin has poor thermal stability.
To solve the coloration problem of the polyester resin, which originates from the polymerization catalyst, yellowish coloration is generally suppressed by adding a cobalt compound to the polyester. Although the color tone (color value b) of the polyester can be certainly improved by the addition of the cobalt compound, there arises a known problem that the melt thermal stability of the polyester is further lowered by the addition of the cobalt compound, and thus accelerating the decomposition of the polymer.
Japanese Examined Patent Publication (Kokoku) No. 48-2229 discloses titanium hydroxide as the other titanium compound used in the polycondensation catalyst of the polyester resin, while Japanese Examined Patent Publication (Kokoku) No. 47-26597 discloses to use α-titanic acid as a catalyst for production of a polyester. However, in the former method, titanium hydroxide is not easily powderized, while in the later method, α-titanic acid is easily changed in properties, and, therefore, it is not easy to store and handle. Therefore, these processes are not suited for use in an industrial field and it is also difficult to obtain a good color tone (color value b).
Furthermore, Japanese Examined Patent Publication (Kokoku) No. 59-46258 discloses to use a product obtained by reacting a titanium compound with trimellitic acid as a polycondensation catalyst for production of a polyester, and Japanese Unexamined Patent Publication (Kokai) No. 58-38722 discloses to use a product obtained by reacting a titanium compound with a phosphite ester as a polycondensation catalyst for production of a polyester. Although the thermal stability of the melt of the polyester is improved to some extent by using these polymerization catalysts, the resulting polymer has poor color tone (color value b) and thus a further improvement in color tone (color value b) of the polyester is required. It is an effective means to use no antimony as the catalyst in order to suppress the deposition of spinneret foreign matters. However, according to the process using no antimony, the resulting polyester resin and polyester resin product, particularly polyester fibers, have unsatisfactory color tone. Therefore, it has conventionally been difficult to put a catalyst free from antimony into practice.
Furthermore, there is known a process of regenerating a polyester resin using, as a raw material, an aromatic dicarboxylate ester obtained by recovering used polyester products (for example, fibers, films and bottles), followed by washing, grinding and further depolymerization. In this case, it is also required to develop a process for producing a regenerated polyester having good transparency and good color tone.